The invention relates to a termination device for a transmission line in which it is desired to reduce to a minimum the standing wave ratio resulting from the reflection of microwaves on a resistive load placed at the end of the line.
Such resistive loads, whose value is equal to the modulus of the characteristic impedance of the transmission line, are often produced in the form of a deposit made on an insulating material, particularly a nickel-chrome alloy film deposited on an insulating ceramic.
This method is of interest in the case of microstrip lines, e.g. in the production of directional couplers in which there is a so-called decoupled channel in which all the ultra-high frequency energy has to be absorbed, even in frequency bands up to 20 GHz.
It is also applicable to lines with two earth planes or striplines, as well as to coplanar lines.
In all cases the absorbing load must meet two requirements:
1. have an impedance, whose real part is equal to the characteristic impedance of the line; PA0 2. have an imaginary part which is as close as possible to zero.
The first condition can easily be satisfied in the case of loads deposited by using a conventional process, which can be adjusted by erosion using a sand jet or by etching using a laser beam.
It is more difficult to satisfy the second condition because capacitive or inductive effects occur due to the substantial surface area and to the irregularities of the nickel-chrome film. Neither the width, nor the length, nor the two dimensions of this surface can be reduced without certain disadvantages occurring. Thus, a thinner film of nickel-chrome, i.e. which is more resistive and consequently has a smaller surface, is unable to withstand certain thermal dissipations, which limits the power behaviour of the device. The film of normal thickness, but e.g. narrower and longer to have the same surface would give a discontinuity or gap and consequently an energy reflection, producing undesirable standing waves at the transition point between the microstrip line conductor and the resistive film.